Its "core" is open source David - but it relies on in-built and included OSX technologies to run - hence licensing costs I guess. Not sure that anyone striving for a better way needs to be accused of trying to make a "killing". I wish them very well - Adobe has been getting my money (and LOTS of it) for years!
"Open source" is probably a bit of a misnomer here - you can contribute to the project and by all accounts so far they really listen to users input - but I'm not sure if you have access to the source code as such. Maybe you do, I haven't dug any deeper yet.
Compared to GIMP2? Delightful.
...I forgot to mention that it opens and saves layered Photoshop files.